The invention relates to a zeolitic material for fluid catalytic cracking, and methods for its preparation and use and, more particularly, to a zeolitic material having a low ratio of silica to alumina which is prepared without the use of organic templates or seeding and which possesses a better cracking activity and a higher selectivity to gasoline and other light fractions or distillates.
A zeolite is a crystalline aluminosilicate containing zeolitic water, which has an oxide molar composition represented by the following general formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.YSiO.sub.2.XH.sub.2 O
wherein M stands for a metal cation, n stands for the valence number of the metal cation M, Y is the molar ratio of SiO.sub.2 to Al.sub.2 O.sub.3 and is generally at least 2, and X is the molar ratio of H.sub.2 O to Al.sub.2 O.sub.3 and is a number larger than 0.
The basic structure of the zeolite comprises SiO.sub.4 tetrahedrons having four oxygen atoms at apexes with the silicon atom being at the center, and AlO.sub.4 tetrahedrons having four oxygen atoms at apexes with the aluminum atom being at the center, where these SiO.sub.4 tetrahedrons and AlO.sub.4 tetrahedrons are regularly and three-dimensionally connected to one another while owning oxygen atoms jointly. Since aluminum atoms are trivalent, each AlO.sub.4 is negatively charged. This negative charge is balanced by cations M.sup.+n to preserve electroneutrality. A three-dimensional network structure having pores differing in size and shape according to the manner of connection of the tetrahedrons can be provided. The thus-formed pores have a size of 2 to 10 angstroms or more, and the pore size can be changed by exchanging the metal cations connected to the AlO.sub.4 tetrahedrons with other metal cations having a different size.
Mordenite Framework Inverted (MFI) type zeolites such as ZSM-5 and ZSM-11 are widely used in refinery processes. These zeolites display exceptional catalytic performance in several reactions such as xylene isomerization, benzol alkylation, and the processing of methanol into gasoline and olefins.
Normally, zeolites are prepared using an alkali metal cation and an organic nitrogen containing compound as a specific organic alkyl-amonium ion. Synthesis of known zeolites such as ZSM-5 and ZSM-11 is expensive because this preparation requires large quantities of amines or organic ammonium salts and special reactor material for supporting the corrosive effects of these materials, as well as their disposal. Otherwise for the use of these zeolites as catalysts or absorbent, organic material inside pores and channels of the zeolite must be removed, requiring an additional process to eliminate organics.
It is therefore desirable to prepare zeolites without the use of organic templates or mineralizers.
U.S. Pat. No. 4,257,885 to Grose et al. discloses a zeolite prepared free of organic cations by using a colloidal silica as the silicon source. Nucleating agents or "seeds" are added during the crystallization procedure to produce a product of sufficient purity and in a sufficient yield. The disclosed procedure results in a zeolitic material having a mole ratio of silica to alumina in the range of 10-100.
U.S. Pat. No. 4,562,055 to Arika et al. discloses a process for the preparation of zeolites similar to ZSM-5. The disclosed procedure provides a zeolite similar to ZSM-5 which has a high purity and a high silica to alumina ratio. Preparation of this zeolite, however, requires preparation of a homogeneous phase compound of a granular amorphous aluminosilicate in an aqueous solution of an alkali metal hydroxide and/or an alkali metal silicate.
European Patent No. 94693Bl to Onodera et al. discloses a method for preparation of ZSM-5 zeolite without organic templates. A seeding procedure is used to expedite the formation of the desired crystalline structure.
All of the above-described zeolitic materials have high silica to alumina ratios. It has been discovered, however, that a low silica to alumina ratio is desirable. Such a low ratio provides a high density of active sites and, consequently, a high conversion. This low silica to alumina ratio characteristic results in an improved ion exchange capacity which is ideal for use in a polar adsorbent molecular sieve type material. A low ratio silica to alumina also appears to be helpful in providing a zeolite structure having desirable shape selectivity properties.
The above-described methods for preparation of zeolitic materials also include undesirable steps of seeding, long periods of crystallization or homogeneous starting materials to produce a ZSM-5-like substance.
Accordingly, it is the principle object of the present invention to provide a highly pure zeolite having an XM.sub.2 O.Al.sub.2 O.sub.3.YSiO.sub.2.ZH.sub.2 O system, wherein M represents an alkali metal cation, X is the molar ratio of the alkali metal cation to Al.sub.2 O.sub.3, Y is the molar ratio of SiO.sub.2 to Al.sub.2 O.sub.3, and Z is the molar ratio of H.sub.2 O to Al.sub.2 O.sub.3, and wherein Y is lower than other typical ZSM-5 or ZSM-11 materials.
It is a further object of the present invention to provide a method for preparing a zeolite of MFI type, wherein organic templates, seeding or homogeneous starting solutions are not required.
It is a still further object of the present invention to provide a zeolitic catalyst having good cracking qualities, and an improved selectivity to olefins.
It is still another object of the present invention to provide a zeolitic material which possesses a high ion exchange capacity.
Further objects and advantages of the present invention will appear hereinbelow.